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Summary1 Freshwater habitats in cultivated and densely populated lowland regions of Europe have experienced profound changes during the last 100 years. We take advantage of the long interest in aquatic plants in Denmark to compare the submerged¯ora in lakes and streams in 1896 and 1996. 2 Most of the lakes which contained a diverse submerged vegetation 100 years ago now have the high phytoplankton biomasses and summer transparencies below 2.0 m characteristic of eutrophication. The majority of 17 lakes included in both old and recent studies have lost all or most of their submerged species. Species richness for those lakes that were vegetated did not, however, dier signi®cantly between old and recent studies. 3 Species richness declined markedly in the 13 streams included in both studies. Over all sites, there was also a signi®cant decline of species richness per locality. Potamogeton species declined from 16 to 9, despite an 8-fold increase in the number of sites surveyed. 4 Similar compositions and rank-abundances of Potamogeton species in lakes and streams studied 100 years ago re¯ect suitable growth conditions and mutual exchange of propagules. Today, low habitat diversity and frequent disturbance in streams and low recruitment from lakes favours only robust, fast-growing species capable of regrowth following weed cutting and dredging. 5 A positive interspeci®c relationship observed in the contemporary stream vegetation between mean local abundance and number of occupied sites was probably promoted by redistribution of plants as a result of disturbance and ecient dispersal in the interconnected stream network. 6 The freshwater macrophyte¯ora in north-west Europe presently includes a high proportion of rare species which are threatened by extinction. Both species typical for oligotrophic conditions (e.g. P. ®liformis and P. polygonifolius) and another group of large, slow-growing species (e.g. P. alpinus, P. lucens, P. praelongus and P. zosterifolius), were once common but are now infrequent, while other transient species have remained rare (e.g. P. acutifolius, P. colouratus, P. densus and P. rutilus). The presence of many species that barely survive in small and distant populations will make re-assembly of submerged aquatic communities dicult.

1. A sediment core from the shallow, hypertrophic Lake Søbygaard (mean depth ∼1 m; [TP] 310 μg P L−1) was analysed for subfossil remains to reconstruct chironomid community changes in relation to the succession and disappearance of aquatic macrophytes. 
2. Species composition in the 1.10 m core indicates a succession from a ‘naturally’ eutrophic state to a hypertrophic state during recent centuries. Radiometric dating (210Pb) of the uppermost 20 cm of the sediment core (∼1932–93) indicates that sediment accumulation rate had doubled in recent decades. 
3. Changes in chironomid assemblages were in close agreement with changes in both diatoms and macrophyte remains in the same core. Distinct changes in chironomid communities reflect the eutrophication process and macrophyte succession through Chara, Ceratophyllum and Potamogeton dominance to the present state, with complete loss of submerged vegetation and dominance by phytoplankton. 
4. The co‐occurrence and relationship between aquatic macrophyte diversity and recent subfossil chironomid assemblages were assessed from an additional 25 Danish lakes. There was good agreement between the macrophyte and chironomid‐based lake groupings. Overall, a significant difference (P<0.001) was found in chironomid assemblages among lakes in different macrophyte classes. In a pair‐wise comparison, the poorly buffered mesotrophic lakes and the alkaline eutrophic lakes had significantly different chironomid assemblages. 
5. Chironomid taxa commonly reported to be associated with macrophytes (Cricotopus, Endochironomus and Glyptotendipes) were shown also to be indicators of highly productive lakes lacking abundant submerged vegetation.

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Unlike in land plants, photosynthesis in many aquatic plants relies on bicarbonate in addition to carbon dioxide (CO2) to compensate for the low diffusivity and potential depletion of CO2 in water. Concentrations of bicarbonate and CO2 vary greatly with catchment geology. In this study, we investigate whether there is a link between these concentrations and the frequency of freshwater plants possessing the bicarbonate use trait. We show, globally, that the frequency of plant species with this trait increases with bicarbonate concentration. Regionally, however, the frequency of bicarbonate use is reduced at sites where the CO2 concentration is substantially above the air equilibrium, consistent with this trait being an adaptation to carbon limitation. Future anthropogenic changes of bicarbonate and CO2 concentrations may alter the species compositions of freshwater plant communities.

We examined the relationship between environmental factors and the richness of submerged macrophytes species in 73 Danish lakes, which are mainly small, shallow, and have mesotrophic to hypertrophic conditions. We found that mean species richness per lake was only 4.5 in acid lakes of low alkalinity but 12.3 in lakes of high alkalinity due to a greater occurrence of the species-rich group of elodeids. Mean species richness per lake also increased significantly with increasing Secchi depth. No significant relationship between species richness and lake surface area was observed among the entire group of lakes or a subset of eutrophic lakes, as the growth of submerged macrophytes in large lakes may be restricted by wave action in shallow water and light restriction in deep water. In contrast, macrophyte species richness increased with lake surface area in transparent lakes, presumably due to expansion of the area colonised by submerged macrophytes. Thus, the size of the colonised area is a better predictor of species richness than lake surface area. The strong increase in species richness accompanying greater transparency can be accounted for by the combined effect of higher colonised area and higher habitat richness along gradients of deeper macrophyte growth.Résumé : Nous avons examiné la relation entre les facteurs environnementaux et la diversité des espèces macrophytes immergées dans 73 lacs danois, qui sont principalement petits et peu profonds et présentent des conditions mésotrophes et hypereutrophes. Nous avons découvert que la diversité moyenne des espèces par lac n'était que de 4,5 dans les lacs acides de faible alcalinité, mais de 12,3 dans les lacs de forte alcalinité, en raison de la forte présence d'élodées. La diversité moyenne des espèces par lac augmentait fortement avec la transparence mesurée au disque de Secchi. Nous n'avons noté aucune relation entre la diversité des espèces et la superficie de la surface, ni dans les lacs ni dans le sous-ensemble de lacs eutrophes, la croissance des macrophytes immergés dans les grands lacs étant peut-être limitée par l'action des vagues dans les eaux peu profondes et par la faible luminosité des eaux profondes. Par contre, la diversité spécifique des macrophytes augmentait avec la superficie de la surface dans les lacs transparents, sans doute à cause de l'expansion de la zone colonisée par les macrophytes immergées. Par conséquent, la superficie de la zone colonisée est un meilleur prédicteur de la diversité des espèces que la superficie de la surface des lacs. L'accroissement important de la diversité des espèces dans les lacs transparents peut s'expliquer par l'effet combiné de la grande superficie colonisée et de la diversité supérieure des habitats selon les gradients de colonisation en profondeur par les macrophytes.[Traduit par la Rédaction] Vestergaard and Sand-Jensen 2031